Messages in this thread |  | | Date | Mon, 13 Apr 2015 11:42:53 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v5 05/10] seqlock: Better document raw_write_seqcount_latch() |
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On Mon, Apr 13, 2015 at 11:21:46AM -0700, Linus Torvalds wrote: > On Mon, Apr 13, 2015 at 10:43 AM, Paul E. McKenney > <paulmck@linux.vnet.ibm.com> wrote: > > > > A shorthand for READ_ONCE + smp_read_barrier_depends() is the shiny > > new lockless_dereference() > > Related side note - I think people should get used to seeing > "smp_load_acquire()". It has well-defined memory ordering properties > and should generally perform well on most architectures. It's (much) > stronger than lockless_dereference(), and together with > smp_store_release() you can make rather clear guarantees about passing > data locklessly from one CPU to another. > > I'd like to see us use more of the pattern of > > - one thread does: > > .. allocate/create some data > smp_store_release() to "expose it" > > - another thread does: > > smp_load_acquire() to read index/pointer/flag/whatever > .. use the data any damn way you want .. > > and we should probably aim to prefer that pattern over a lot of our > traditional memory barriers.
I couldn't agree more!
RCU made a similar move from open-coding smp_read_barrier_depends() to using rcu_dereference() many years ago, and that change made RCU code -much- easier to read and understand. I believe that moving from smp_mb(), smp_rmb(), and smp_wmb() to smp_store_release() and smp_load_acquire() will provide similar maintainability benefits. Furthermore, when the current code uses smp_mb(), smp_store_release() and smp_load_acquire() generate faster code on most architectures.
Thanx, Paul
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